Laminar flow is the smooth, uninterrupted flow of air over the contour of wings, fuselage, or other parts of an aircraft in flight. Maintenance of laminar flow during cruise may become an operational necessity for future fuel-efficient aircraft configurations. Laminar flow can be maintained in the streamwise direction on an aircraft wing either passively through natural laminar flow induced by the shape of the airfoil, actively by laminar flow control through perforations on the airfoil that can introduce suction, or by hybrid laminar flow control which combines the two approaches. Factors that can destabilize a laminar boundary layer and cause transition to turbulent flow include adverse pressure gradients, surface roughness, heat, and acoustic energy. In the case of surface roughness, the critical height of a topographical imperfection that induces transition from laminar boundary layer flow to turbulent is dependent on the airfoil and Reynolds number and can be as small as several microns. Flight tests have shown that insect strikes on wing leading edge surfaces can leave residue exceeding the critical heights sufficient to disrupt laminar flow and decrease fuel efficiency. These residues have long been recognized to adhere to exposed aircraft surfaces. Specifically, the drag coefficient measured on an aircraft wing was determined to increase as much as 100% according to previous studies. Studies have shown that airborne insect densities are greatest between ground level and 153 m, with the highest insect population present during conditions of light winds (2.6 to 5.1 m/s), high humidity, and temperatures ranging from 21° C. to 29° C. As a consequence, aircraft are most susceptible to insect strikes during taxi, takeoff, initial climb, approach, and landing. Similarly, insect debris can influence the efficiency of wind turbines.
The development of surface roughness from insect strikes involves numerous complex chemical reactions. A comprehensive review of the relationships between airflow, insect strike location, and the resultant insect residue heights was published in Coleman, W. S. “Boundary Layer and Flow Control”, ed. G. V. Lachman, Pergamon Press, 1961 (“Coleman”). Based on Coleman's report, impacts from insects during air travel are most likely to occur on the leading edge and immediately surrounding area of an aircraft wing. Thus, natural laminar flow would be interrupted, diminishing fuel burn rate improvements arising from airfoil shape, as well as, negating further efficiency improvements from hybrid laminar flow systems. Numerous approaches to mitigate insect residue adhesion on the wing leading edge surface have been investigated over the past 60 years. The easiest and most economical of these approaches relied upon the natural erosion of insect residue through a combination of air temperature, flight speed, and moisture provided by flying through clouds. However, this approach is too condition-dependent to be reliable. In other approaches, hardware-based solutions have included mechanical scrapers, deflectors, paper, and/or other coverings. While these technically solved the problem, these approaches were either difficult to implement and/or necessitated a weight penalty preventing implementation on a commercial scale. As another example, the Krueger flap is a deflector designed to improve lift for large aircraft (e.g. on the Boeing 737 and the Boeing 747) during takeoff. Although it has been shown to negate insect residue from the wing leading edge so as to retain laminar flow on the upper wing, physical discontinuities of the flap may induce early boundary layer transition on the lower wing surface.
Physical and chemical modifications to the wing leading edge surface have been investigated including elastic surfaces, coatings, soluble films, and fluid covers. Elastic surfaces were found to work well with minimal traces of insect residue, but rain erosion and hail were a concern due to potential damage to the surface. Soluble films, such as glycerin, provided a good barrier to insect residue adherence to the wing surface and could be easily washed or blown away taking the insect residue with it. Problems with this strategy were that it was only useful upon takeoff, had to be applied prior to every flight, and if the film did not provide complete coverage (i.e. wetting) over the wing then insect residue would stick to the non-wetted wing surfaces. Fluid covers formed through a continuous liquid discharge from the deicing system were successful in preventing a majority of insect residue from adhering to the surface; however, it was only effective when turned on. Besides coverage and environmental concerns, weight and economic penalties associated with transporting the fluid necessary for the portions of the flight profile where insect strikes are a problem as described above could be problematic, as well as complete coverage of the wing surface by the fluid as previously mentioned.